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tl;dr: They do. It's just harder to tell how much.
The longer answer is that they do and that effective compression is failing you as an approximation for actual effects.
Think about detonation (AKA premature ignition of the fuel-air mixture). Normally we consider two causes: compression (the change in the space enclosed by the cylinder as the piston ...

First... find another garage. White smoke and low compression are NOT symptoms of a bent connecting rod. Also, you CANNOT inspect the connecting rods by removing the head - they're on the opposite (bottom) ends of the pistons.
White smoke and low compression are both symptoms of a blown head gasket. That's very classic. It doesn't sound like it's very bad, ...

One of the reasons that a turbo setup with the equivalent effective compression is more forgiving of low octane gas than than a static compression setup is that you're not at that compression ratio all the time. Take that honda, for example. At 9:1 static ratio, you can run 87 octane all day as long as you don't push any boost at it. When you do start ...

Yes it can. The reason is, these are two different tests, done differently, reading different things. To understand this, you need to understand how the tool works for each test.
For the compression test, the test apparatus captures and holds the compression as it builds through several, but counted revolutions of the crankshaft. You will get so much ...

Wow! Textbook case of worn rings. The oil helps seal the ringland area. That's a large increase on cylinders 2, 3, and 4. Your fuel efficiency should be pretty poor with those compression numbers. Is it?
If your fuel efficiency is miserable, it's time for a rebuild. Depending on the car it might be cheaper just to replace the engine with one from a engine ...

Measure each cylinder and buy over sized pistons / rings to accommodate the sizes.. this will be your cheapest option because installing new liners and buying stock pistons is very expensive / allot of machining work.
If you don't have the time or money, keep driving it the way it is - if its not smoking bad you should get another 35k or more off of it.

In addition to good answer by @Bob:
There are some tricks that can be used to ease the problem:
A knock sensor for detecting premature detonations (and adjusting boost pressure). E.g. Saab APC allows safe use of lower octane fuels.
Injecting water to cool the combustion chambers (instead of excessive fuel)
Per cylinder exthaust thermometers (and ...

Yes, a warped lower end can affect the compression test. A seriously warped head of block can lead to compression leakage through the small space between the head gasket and the head or block. But I don't think you have a bend block, I suspect you used your old head with a new head gasket? You need to have a specialized company 'flatten' the head. I don't ...

To make sure I answer correctly let me make sure I understand the question.
The engine was using 1.5 L of oil per 2000km, you did the following repairs to the engine:
Valve job including replacing the exhaust valve.
Bored the engine 0.5mm over
New 0.5 mm over piston and rings
Now that the repairs have been made it seems to be fixed. It's running well and ...

If the specs call for 91 octane, use 91 octane. Using higher octane fuel will not give you better performance. Only engines which need the higher octane will see better performance from the higher octane. In most cases, engines which should use the lower octane will get worse gas mileage from the higher octane fuel.

Rings generally wear at the same rate meaning that when it's time to replace one you should be replacing them all as they're not seating with the cylinder properly any more.
Burning oil does not always imply a loss of compression. Cars with good rings will burn oil due to bad valve guides or bad valve guide seals.
A compression test should help you ...

As far as I'm aware, it cannot be done. Vacuum is a direct relation to valve lift and timing, does not relate to compression ratio. The vacuum is drawn during the intake stroke. In a 4-stroke (cycle) engine, the intake and compression events are completely separate.

If you don't get heat into your cabin, it's usually because of a blocked radiator. That would explain your overheating too. Unfortunately, that ALSO means that you could have damaged your head gasket, which would explain the low compression and difficulty starting.
They don't sound particularly shady to me. Given your history and your current problem with ...

Oil on the spark plug is likely either because the piston rings are not sealing or the valve guides are leaking oil. Both could also be related to low compression. You could either have ring seal problems or a valve that is not seating properly. Valve seat problems could be due to physical damage or carbon build-up coming loose. Guess at what is most likely ...

There's a wide variation, but back in the '90's it was clustered in the 10-12psi range for a typical 4-cyl turbocharged performance model. Nowadays 16-18psi is not abnormal for high performance factory cars. Of course, you'll find some economy cars in the 5-6psi range, where they're using boost to let a little tiny fuel-efficient engine not be completely ...

This question doesn't provide enough information for us to answer it well as originally written. Some of the important factors determining what boost level is used include:
Base compression rate of the engine (see this question for related discussion).
Octane of the fuel to reduce the risk of detonation (see point 1).
The temperature and humidity of the ...

If the bore has been honed and rings replaced then perhaps the issue is with the valve stems. I once had a CBX250 bike that blew a lot of smoke. It was a 4-stroke, like yours. In the end, it turned out the value stems were leaking oil into the cylinder.
Does your bike produce a lot of smoke?